In general, a magnetoresistive sensor uses the magnetoresistance of ferromagnetic materials, that is to say the variation in the electrical resistance of a conductor under the effect of the magnetic field applied thereto. In practice, this type of sensor requires the application of an excitation current. The output voltage obtained depends on the excitation current and on the magnetoresistance and makes it possible to read out the value of the applied magnetic field. Depending on the sensors, this measurement is a normal measurement, that is to say in a direction orthogonal to the direction of the current, or a transverse measurement, that is to say in the plane of the sensor (collinear or perpendicular to the direction of the current).
The criteria employed for comparing the performance of various types of sensor may typically be: the smallest magnetic field measurable at a given frequency (typically 1 Hz), the mass, the bandwidth, the dimensions of the sensors and their consumption, when they are connected to electronics for conditioning the signal. The preference for these criteria differs according to the application. For CETP (Centre for Terrestrial and Planetary Environmental Studies), which relate to the magnetic component of waves in space plasmas, it is envisaged producing sensors that are sensitive to magnetic fields with a sensitivity of the order of 100 pT/√{square root over (Hz)} at 1 Hz and have a low mass and a low consumption in order to be on board.
Among the technologies used for producing this type of magnetometer, the following may in particular be listed:                Hall-effect sensors: small size and low consumption, but it may be difficult for their sensitivity to reach 100 nT/√{square root over (Hz)};        AMR (anisotropic magnetoresistance) and “AMR derivative” sensors, GMR (giant magnetoresistance) sensors and TMR (tunnel magnetoresistance) sensors: small size and low consumption; their sensitivity may reach 1 nT/√{square root over (Hz)} at 1 Hz, TMR sensors representing a promising technology leaving a sensitivity of 10 pT/√{square root over (Hz)} at 1 Hz conceivable;        GMI (giant magnetoimpedance) sensors: these have a low consumption; the other characteristics are still poorly understood since these components are the subject of a recent return of interest, although their principle is well known. The possibility of miniaturization and evolution hint at a low-mass potential and sensitivities below 1 nT/√{square root over (Hz)};        flux gate sensors (or flux valve sensors): by far the most common, their size determining their sensitivity. For sensors weighing a few tens of grams, a sensitivity of 100 pT/√{square root over (Hz)} is achieved. The technology has reached a degree of maturity which leaves no hope of improving their performance;        SQUID sensors: sensors using the Josephson effect; this type of sensor requires a superconducting ring cooled with liquid helium, offering a sensitivity possibly of less than 1 pT/√{square root over (Hz)}; it is mainly used in medicine. Its mass and its consumption are unacceptably high in many applications; and        Search coil sensors (or flux meters): very sensitive (close to 10 fT/√{square root over (Hz)} at 1 kHz) and of low consumption, but bulky (cube of 10 cm per side). These sensors rely on the phenomenon of induction and in principle their excellent sensitivity for frequencies above a few Hz rapidly deteriorates when the frequency drops and does not allow slowly variable magnetic fields to be measured (at less than 1 Hz).        
At the present time, the requirements in magnetic sensors with a typical precision of 10 pT are solved using flux-gate technologies. However, these technologies are bulky, not integrateable and relatively expensive in terms of energy. This is why alternatives to this type of magnetic sensor are sought.
The sensitivity of certain types of sensor (in particular Hall sensors) has been able to be improved by having magnetic cores at the centre of which the sensor is positioned. The magnetic amplification thus obtained depends on the distance between the two cores, the smaller this distance the higher this amplification. It has thus been demonstrated that the amplification could locally exceed the relative permeability of the magnetic material and reach amplification factors of more than 1000, when the distance between the two cores is of the order of 100 μm. (P. Leroy, C. Coillot, A. Roux and G. Chanteur, “Optimization of the shape of magnetic field concentrators to improve sensitivity of Hall sensors”, proceedings of the SSD, 05 Congress in 2005). In the case of Hall-effect sensors, sensitivities of the order of 100 pT/√{square root over (Hz)} at 1 Hz may be envisaged. However, this type of amplification is possible for sensors in which the thickness of the “sensitive” region can be reduced substantially (a few tens of microns), something which is more difficult to achieve in the case of AMR, GMR, TMR and GMI sensors, for which the sensitivity to the magnetic field is in the transverse direction. This is because an AMR chip may be likened to a parallelepiped (with dimensions of the order of 2 mm×2 mm×0.1 mm). In this case, the minimum distance between the cores is equal to the width of the chip (2 mm in the aforementioned example) and the magnetic amplification is then low since the distance between the cores is large.